Re-oxygenation has been reported after treatment with the clinically used anti-neoplastic agent cyclophosphamide (CTX). In this project, the timing of a drop in pO2 and slower reoxygenation has been studied. Within one hour post, a single dose of CTX (100 mg/kg) intratumoral pO2 drops significantly (3-4 torr).Rapid re-oxygenation occurs with a return to pretreatment levels 3 hours post CTX. Further re-oxygenation occurs reaching a maximum 72 hours post treatment. Information about the effects of anti-neoplastic agents on tumor pO2 will b e exploited to study timing of effective drug and radiation combinations.